To make the most of limited water resources, a system for purifying and re-utilizing wastewater discharged from buildings, residences, and the like has been known. For instance, Patent Document 1 discloses a wastewater re-utilizing system configured to use drainage of clean water used in a household or the like and rainwater for flushing toilets to save water. Further, Patent Document 2 discloses an interior greening system for utilizing gray water, which produces gray water by treating wastewater in a building and re-utilizes the produced gray water for watering plants that grow inside a building.